Metastasis, i.e. the spread of cancer from a primary site to non-adjacent secondary sites is one of the major challenges of cancer research, diagnosis and treatment. Metastasis is believed to develop through a series of steps including localized invasion, intravasation, transport through circulation, arrest in microvessels, extravasation, formation of micrometastasis, dormancy, angiogenesis, colonization and formation of macrometastasis (Weinberg, 2007, The biology of cancer, Garland Science, New York). During their transport in the circulatory system, in particular in the blood stream, the tumor cells are referred to as circulating tumor cells (CTC). CTC are considered to play a critical role in the metastatic spread of carcinomas, i.e. of malignancies of the epithelium. Their detection has thus important prognostic and therapeutic implications and is associated inter alia with clinical stage, disease recurrence, treatment response and patient survival. CTC are further seen as independent surrogate markers for the assessment of risk of relapse and the course of treatment.
However, CTC are extremely rare in the bloodstream and are found only with a frequency of 1-10 cells per ml of whole blood in patients with metastatic diseases, in comparison to 109 red blood cells and several million white blood cells, which can be found in the same volume. It is therefore necessary to select and enrich these rare circulating tumor cells. Traditionally, the enrichment process was based on density differences of the CTC, which were separated by density centrifugation. Though, recovery rates are very poor. More recently, the enrichment of CTC was based on the identification of specific surface markers, in particular epithelial adhesion molecule (EpCAM). CTC can accordingly be enriched immuno-magnetically, e.g. by means of ferrofluidic nanoparticles. Yet, this approach is hampered by a broad recovery range of about 10 to 90%, which is apparently due to the variable expression of surface markers (Zheng et al., 2011, Biomed Microdevices, 13: 203-213).
As alternative for the isolation of CTC the use of cell size exclusion methods has been proposed, in particular due to the fact that CTC are as epithelial cells in many cases larger than the surrounding blood cells. Several filter types have been developed including polycarbonate filters comprising pores at random locations, microfabricated filters, microfabricated single layer 2D filters, microfluidic filters based on electric fields, filters microcavity array devices based on nickel electroforming or 3D microfilter devices (Zheng et al., 2011, Biomed Microdevices, 13: 203-213). Similar approaches have also been developed for the selection of other cell types, e.g. bacterial cells within the blood stream or within certain sample forms, bone marrow cells or spleen cells etc.
However, the selection of specific cells from a solution is only the first step of a sequence of reactions necessary to determine the cells' properties, e.g. their metastatic characteristics. Thus, after having been isolated, the cells are typically subjected to staining and optical differentiation procedures. The filter material during these subsequent steps is generally difficult to handle, thin, fragile, and may break easily.
In addition, filters are often corrugated and not optically flat, thus complicating the process of optical imaging of the cells. Although the prior art provides approaches to overcome this problem, e.g. by using immuno-magnetic beads and magnets to pull cells against a microscope cover, the methods are complex, time-consuming or necessitate specialized equipment.
There is hence a need to provide means and methods, which allow for handling of filter material comprising isolated cells such that the captured cells can rapidly and accurately be imaged on the filter material.